Conventionally, a laser diode or a light-emitting diode has commonly been used as a pumping light source of a fluorescent microscope or a light source of a semiconductor exposure device or an optical characteristic evaluation device, because of its small size and low power consumption.
However, the laser diode and the light-emitting diode have temperature dependence, and their luminance wavelengths and intensities vary with variations in temperature. Therefore, there is proposed a temperature control apparatus comprising a heating/cooling means such as a Peltier device and a temperature detection means such as a thermistor, and controlling a light-emitting device (laser diode or light-emitting diode) to keep the temperature thereof constant (for example, Japanese Published Patent Application No. Hei. 7-22678).
FIG. 5 is a block diagram illustrating conventional temperature control apparatus.
The temperature control apparatus shown in FIG. 5 comprises a light-emitting device 500, a photodetector 501 for outputting a light intensity signal of the light-emitting device 500, a light-emitting device controller 502 for outputting an optical output control signal for controlling an optical output from the light-emitting device 500 on the basis of the light intensity signal, a Peltier device 503 for setting the temperature of the light-emitting device 500 to a predetermined target temperature, a temperature detector 504 for detecting the temperature of the light-emitting device 500, a reference voltage holding unit 507 for holding an output voltage of the temperature detector 504 corresponding to the target temperature, a difference amplifier 508 for amplifying a difference between the output voltage of the temperature detector 504 and the output voltage of the reference voltage holding unit 507, and a Peltier to controller 509 for controlling the Peltier device 503 according the output of the difference amplifier 508.
The difference amplifier 508 is implemented by a differential amplifier circuit using an operational amplifier.
Further, the temperature detector 504 comprises a resistor 505 and a thermistor 506 which are connected in series between the power supply and the ground.
The thermistor 506 is a device whose resistance value varies with variations in temperature. When a thermistor having a B constant of 3380±1% and a resistance value of 10K±1% Ω at 25[° C.], and a resistor of 10KΩ are connected in series between a 5V power supply and the ground, the output voltage of the temperature detector 504 varies as shown in table 1. The output voltage becomes 2.5[V] at 25[° C.], and decreases by about 40 [mV] as the temperature increases by 1° C.
TABLE 1temperature of light-emittingoutput voltage of temperaturedevice [° C.]detector [V]103.2202.7252.5302.3401.8501.5601.2
For example, since the light-emitting device 500 can be controlled to 25[° C.] by keeping the output voltage of the temperature detector 504 at 2.5[V], a controller (not shown) of the whole system including the temperature controller sets a reference voltage at 2.5[V] on the reference voltage holding unit 507. When making the light-emitting device 500 emit light, a current supplied to the light-emitting device 500 cannot be converted into optical energy by 100%, and the electric energy that has not been converted into optical energy becomes thermal energy. At this time, since the temperature of the light-emitting device 500 increases and thereby the output voltage of the temperature detector 504 decreases, a difference between the output voltage of the temperature detector 504 and the output voltage of the reference voltage holding unit 507 is amplified by the difference amplifier 508, and the Peltier controller 509 applies a driving current to the Peltier device 503 on the basis of the amplified difference voltage so as to cool the light-emitting device 500. The above-mentioned operation enables the temperature of the light-emitting device 500 to be kept at 25[° C.].
On the other hand, when the temperature surrounding the light-emitting device 500 decreases and thereby the temperature of the light-emitting device 500 decreases, the output voltage of the temperature detector 504 increases. Therefore, the Peltier controller 509 applies a driving current to the Peltier device 503 so as to heat the light-emitting device 500, whereby the temperature of the light-emitting device 500 can be kept at 25[° C.].
Further, there is proposed another temperature control apparatus which is obtained by providing the above-mentioned temperature control apparatus with a means for feeding a voltage corresponding to the driving current applied to the Peltier device back to the input of the difference amplifier to add the voltage to the output voltage of the temperature detector (for example, Japanese Published Patent Application No. Hei. 7-321392). This temperature control apparatus can stabilize temperature control when the temperature changes between cooling and heating, and minimize temperature fluctuations in the critical value of the driving current applied to the Peltier device.
In the conventional temperature control apparatus, however, when the light-emitting device emits light or is quenched, the current flowing in the light-emitting device varies and thereby the ground voltage varies. Therefore, even though the temperature of the light-emitting device does not change actually, the output voltage of the temperature detector varies, whereby the Peltier controller changes the temperature of the light-emitting device to a temperature different from the desired temperature.
Further, if the temperature detector and the light-emitting device share a power supply, the current flowing in the light-emitting device varies when the light-emitting device emits light or is quenched and thereby the power supply voltage varies. Therefore, even though the temperature of the light-emitting device does not change actually, the output voltage of the temperature detector varies, whereby the Peltier controller changes the temperature of the light-emitting device to a temperature different from the desired temperature.
Particularly, it becomes very difficult to realize stable power supply and grounding because of an increase in the driving current of the light-emitting device with an increase in the output power of the light-emitting device in recent years, and a reduction in the size of the light-emitting device module including the temperature detector.
Moreover, when the light-emitting device deteriorates with time, the current value for keeping the same light intensity gradually increases, whereby variations in the power supply voltage or the ground voltage gradually increase.